<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        
    </style>
</head>
<body>
    
</body>
<script>
    
    var a;
    var num = 38 - a;
    console.log(num);            //nan

    console.log( isNaN(num) );    //true
   
    console.log( isNaN(6) );   //false
    
    console.log( isNaN("hello") );   //true
    
    console.log( isNaN("37") );    //false

    console.log( isNaN(true) );   //false


    console.log( undefined === undefined )  //true
    console.log( null === null )   //true
    console.log( NaN === NaN )    //false

    console.log( undefined == null )   // true
    console.log( undefined == NaN )    //false
    console.log( NaN == null )     //false

</script>
</html>